Thickness detectors are well-known in the art of high-speed document processing. For instance, in the field of high-speed automated mail processing, thickness detectors are utilized to detect the thickness of the envelopes being processed. Typically, a batch of mail is placed in an input bin and the envelopes are serially fed from the input bin along a transport path. A thickness detector positioned along a transport path detects the thickness of each envelope. Further processing of an envelope is controlled by the thickness of the envelope measured by the thickness detector. For instance, if an envelope has a thickness that is greater than a predetermined threshold, the envelope is outsorted and processed separately from the envelopes having a thickness below the threshold.
The accuracy, reliability and durability of the known thickness detectors decreases as the speed of the transport conveying the mail is increased. Accordingly, the present invention provides an accurate, durable and reliable thickness detector capable of processing documents being transported at high speeds.